As known, a solenoid valve is an adjustment component of a hydraulic or pneumatic circuit that makes it possible, by means of an electric actuator, to open and close and, more generally, to regulate the passage of a fluid inside the hydraulic circuit. For example, a solenoid valve can be configured to regulate the passage of compressed air between the holes of a pneumatic circuit so as to then be able to optionally command a series of external actuators. The actuator of a solenoid valve normally consists of a slider body that, under the thrusting of an external force of various origin (magnetic, mechanical, piezoelectric, etc.), opens or closes a nozzle.
In certain industrial applications it is required to assemble, on suitable bases, two or more solenoid valves to create a so-called island or battery of solenoid valves. An island or battery of solenoid valves is a set of pneumatic solenoid valves typically consisting of:    a certain number of solenoid valves, each of which comprises an air slide valve inside it, with the relative sealing members, an actuator element generally consisting of a pneumatic piston, configured to move the air slide valve, and an electropilot, consisting of a reel that, with electromagnetic force, moves a core that opens and closes the passage of air to the piston;    one or more bases, which contain the air passage ducts and the electrical power supply for the electropilots. The bases can be fixed to one another in a modular manner, with different systems depending on the manufacturer;    one or more head elements at which the connections for the electrical signals arrive, which are then sorted to the bases, the pneumatic supply ducts and the centralized discharge ducts of the compressed air.
Each solenoid valve is connected in a fixed manner to its base, usually through screws. The design of the solenoid valves and of the bases is carried out so that, at the moment when the solenoid valve is fixed, the air ducts and the electrical connections for the electropilots are connected automatically.
There are solenoid valves with different configurations. These configurations are normally defined by two identifying numbers. The first number indicates the number of air passageways, whereas the second number indicates the number of positions that the air slide valve of the solenoid valve can assume. For example, a “5/2” solenoid valve has five air passageways and two positions of the air slide valve. Hereinafter, purely as an example, some main types of solenoid valves are listed, with the relative operative configurations:    “5/2” solenoid valve with a single electropilot (with pneumatic or spring return);    “5/2” solenoid valve with double electropilot;    two “3/2” solenoid valves housed in the same valve body and in normally open version (in other words, in non-supplied condition of the solenoid valve, the air is able to pass from the inlet ducts to the outlet ducts);    two “3/2” solenoid valves housed in the same valve body and in normally closed version (in other words, in non-supplied condition of the solenoid valve, the air ducts are closed);    two “3/2” solenoid valves housed in the same valve body and with one solenoid valve normally open and the other solenoid valve normally closed;    “5/3” solenoid valve.
FIG. 1 shows a typical solenoid valve according to the prior art, which contains two “3/2” solenoid valves in the same body. Reference numerals 101 and 102 indicate the two air slide valves, otherwise known as spools, whereas reference numerals 201 and 202 indicate the respective control pistons. When the electropilot makes the compressed air reach each control piston 201 and 202, this pushes the respective spool 101 and 102 towards the central hole 300 of the solenoid valve. When the electrical power supply is interrupted, each spool 101 and 102 goes back into rest position pushed by a spring, or pushed by the compressed air that is always present in the central hole 300 of the solenoid valve.
In certain applications there is a need to be able to have both a first group of solenoid valves with a certain flow rate of compressed air, and at least one second group of solenoid valves with a flow rate of compressed air greater than that of the first group of solenoid valves. This requirement can typically be met in two ways:    installing two distinct islands of solenoid valves, wherein a first island comprises small sized valves and the second island comprises large sized valves;    making islands of solenoid valves in which it is possible to house both small sized valves, and large sized valves.
For example, document EP-A-1026430 to the same Applicant illustrates an island of solenoid valves in which it is possible to house both small sized solenoid valves, and large sized solenoid valves. Document EP-A-1729014 illustrates a compressed air dispensing apparatus provided with a device for increasing the flow rate of compressed air without increasing the size of such an apparatus. U.S. Pat. No. 5,606,993 also illustrates a solenoid valve provided with a device for increasing the flow rate of compressed air.
The solution most often adopted to combine solenoid valves of different sizes consists of making modular bases with different width, typically with one base twice as wide as the other, and designing and making solenoid valves with different widths. In a practical embodiment, small bases with four positions and large bases, of equal total width with respect to the small bases, which however contain only two positions, were built. Consequently, such bases are capable of housing two different types of solenoid valves, in which one is double the width of the other.
With respect to the prior art, wherein completely separate small islands and large islands are provided, this embodiment offers substantial advantages in terms of modularity. On the other hand, this embodiment forces different bases and different solenoid valves, small and large, to be designed, manufactured and managed.
The aim of the large solenoid valve is to allow a greater air flow rate. The air flow rate depends on the size of the passage ducts, both inside the base, and inside the solenoid valve. Consequently, making larger solenoid valves and bases makes it possible to increase the size of the ducts. Vice-versa, the smaller solenoid valves, provided with passage ducts of relatively reduced dimensions, are unable to increase their air flow rate.